Stereophonic amplifier



Feb. 25, 1964 H. N. PARKER STEREOPHONIC AMPLIFIER 2 Sheets-Sheet 1 Filed 001;. 19, 1959 l l I I Q m W N% nwmm \NQQQ N A III N f/m M: 7%,",

Feb. 25, 1964 H. N. PARKER 3,122,713

, STEREOPI-IONIC AMPLIFIER Filed Oct. 19, 1959 2 Sheets-Sheet 2 H920; 0 M RQQKEE INVENTOR.

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United States Patent 3,122,713 STEREQRHONKC AWLEFIER Harold N. Parker, Van Nuys, Calif., assignor to Calbest Engineering 8; Electronics Co., Los Angeles, Calif., a corporation of California Fiied (Pct. 19, E59, Ser. No. 847,133 8 Claims. (Cl. 330-122) This invention relates to a plural channel amplifier and particularly to a stcreophonic amplifier.

As discussed in my prior joint application with Herbert L. Fremont, Serial No. 783,015, filed December 26, 1958, and entitled Multiplex Transmission and Reception System for Stereophonic Material, there is a well defined frequency range in which stereo effects exist. Frequencies of about 200 cycles per second and below tend to be in phase at the spaced transmitting microphones, and very little stereo effect, if any, is perceived.

High quality speaker systems for the low frequency ranges being expensive, there is an apparent saving if one speaker system can be provided to cover the range up to about 200 cycles per second. The expected savings, however, have not been fully realized because there are certain complexities introduced. For example, if there are two power amplifiers respectively for a right channel and a left channel, the single low frequency speaker must derive excitation from both power amplifiers if balanced output is to be achieved.

f course, a crossover network can be provided in connection with each amplifier, and the low frequency components, of both the right and the left channels are readily available. The duplication of crossover equipment is, of course, a disadvantage, especially since the components must handle signals at substantial power levels. But more serious disadvantages result from an attempt to mix the components and apply them to the voice coil of the low frequency speaker. The crossover networks then are interconnected. Since they present low impedances to the low frequencies, the result is that one of the amplifiers will tend to drive the other whenever the signals in the chnnels differ, which they virtually always do. Distortions result from saturation of output transformers and the like.

A solution to the foregoing problem has been suggested, and that is to mix the low frequency components of the respective channels acoustically and not electrically. This is accomplished by providing two voice coils on a single low frequency speaker instead of a single-voice coil at which the signals might be mixed. This, too, is expensive. At this point it is questionable whether there are any economies in attempting to provide a single low frequency speaker system.

A primary object of this invention is to overcome the foregoing disadvantages and provide for the first time an economical single low frequency channel free of distortions. To accomplish this arrangement, crossover is accomplished at the low power level, and components of modest rating can be used. Yet no extra amplifier tubes are used to provide power amplification for the low frequency components. Also, isolation of the respective channels despite the mixture of the low frequency components is economically achieved by the aid of low impedance separators in the form of cathode follower circuits.

A further object of this invention is to provide a power amplifier for the low frequency components by using amplifying devices that already comprise a part of power amplifiers for the respective channels. Thus, the push-pull amplifying devices of one of the channels are, as to the low frequencies, virtually connected in parallel for use as one-half of a push-pull arrangement. Similarly the pushpull amplifying devices for the opposite channel are in "ice essence connected in parallel to form the other half of the push-pull device for the low frequency components. Relatively distortionless output is achieved in a unique manner for the low frequency components and without introducing distortions or disrupting the efficient operations of the respective channels for the respective righthand and lefthand signal components. By such means the number of operative tubes is minimized.

This invention possesses many other advantages, and has other objects which may be made more clearly apparent from a consideration of one embodiment of the invention. For this purpose, there is shown a form in the drawings accompanying and forming part of the present specification. This form will now be described in detail, illustrating the general principles of the invention; but it is to be understood that this detailed description is not to be taken in a limiting sense, since the scope of this invention is best defined by the appended claims.

Referring to the drawings:

FIGURE 1 is a diagrammatic view illustrating a circuit in which the present invention is embodied; and

FIG. 2 is a diagram indicating parts of the circuit illustrated in FIG. 1, and as it virtually exists with respect to the low frequency components.

Indicated at the lefthand side of FlG. 1, are two signal sources 10 and 11 corresponding to the left channel and the right channel of a stereophonic system. In the present instance, the signal sources are assumed to include preamplifier circuits.

From the output lead 12 of the signal source 10 a midrange and high frequency components is derived at the lead 14, and similarly from the lead 12 is also derived a lower frequency component at the terminal 16. For this purpose a high pass filter 18 and a low pass filter 20 are provided. These filters have separate outputs 14 and 16, and a common input terminal 22 fed by the source it). Similarly, an output lead 15 and an output terminal 17 are provided in connection with a high pass filter l9 and a low pass filter 21 associated with the opposite channel 11. i

The output leads 14 and 15 from the two channels, and corresponding to the midrange and high frequency components are routed to separate push-pull power amplifier structures 24 and 25'. The filters, being in advance of the power amplification stages, utilize components of only modest rating.

Each power amplifier for example, for the left channel, includes a phase inverter tube 26 of conventional form that provides oppositely phased signals for application to the grids 28 and 30 of push-pull output tubes 32 and 34. The output tubes 32 and 34 in turn drive the primary winding 82 of an output transformer T1. A secondary winding 36 is to be connected to the midrange and high frequency speaker system (not shown) for the left channel. The usual grid resistors 33 and ii? are connected together at a junction 42. In usual phase inverter applications, this junction 42 is directly grounded. However, for reasons that will appear more fully hereinafter, a lead 46 from the junction 42 connects with ground '44- via a condenser 48. The condenser 48 has negligible impedance at the mid-range and higher frequencies with which the power amplifier 24 is intended to cooperate, but a substantial impedance at low frequencies. Hence, for all practical purposes, the junction 42 is grounded as to the amplifier structure described.

Furthermore, in the conventional push-pu1l amplifier, the cathodes usually connect to the common terminal 42 and thereby to ground. The cathodes have separate paths to ground independent of junction 42 for reasons also to appear.

The power amplifier 25 is identical to the power amplifier 24. A connection 4'7 from a junction 43 between grid resistors 39 and 41 leads to the ground connection 45 via a condenser 4-9. A secondary winding 37 from an output transformer T2 connects with the speaker system for the right channel.

The outputs from the low pass filters 29 and 2 and at the junctions i6 and 17, are combined by a lead 59. A common terminal condenser 52 to ground is provided. The combined signal is applied to the input of a phase inverter tube 51 whereby a push-pull system may be used. The lead 50 thus connects to the grid 53 of the inverter tube 51 via a blocking condenser 54. A grid leak resistor 55 common to both channels is provided in the conventional manner.

The essential problem is to prevent the low frequency signal components from the respective channels overpowering and reverse feeding into the opposite channels and thereby creating distortions. In order to solve this problem, the lead 12 from the signal source of pre-amplifier does not connect directly with the terminal 22 which forms a common input to the high and low pass filters 18 and 20. Instead a low output impedance device in the form of a cathode follower stage V1 is interposed therebetween to serve as a means for isolation. A cathode follower stage V2 is similarly interposed between the common terminal 23 of the high and low filters l9 and 21 of the opposite channel of the lead 13 from the signal source or pre-amplifier 11. Thus for the left channel, a triode V1 has its cathode 60 connected to one side of a cathode follower resistor 62. The other side of the cathode follower resistor 52 is connected to a ground lead or return 6-4. The common input terminal 22 for the filters 18 and 20 is connected to the cathode whereby a cathode follower circuit is completed.

The plate 66 of the tube V1 is provided with excitation by a suitable source, indicated by the legend The lead 12 from the signal source or pre-amplifier 10 connects to the grid 68 of the tube V1 and a grid resistor 70 is provided between the grid 68 and the ground connection 72.

Isolation of the low pass filters from the opposite sources being accomplished by the cathode follower stages, it remains for the combined low frequency signal to be amplified to a suitable power level for driving a low frequency speaker system.

The combined low frequency signals are applied to the phase inverter tube 51. Signals of opposite phase exist at the leads 80 and 81 connected to the plate and the cathode of the tube 51 via blocking condensers. A push-pull circuit is utilized for amplification of the low frequency signal existing across the leads 80 and 81. One section of the push-pull system comprises the tubes 32 and 34 operating in parallel, and the other section of the push-pull circuit comprises the tubes 33 and 35 operating in parallel. The tubes 32 and 34 operate in parallel for low frequency signals because the inductance of theprimary winding 82 of the transformer T1 produces negligible impedance at low frequencies. Hence, the plates 84 and 86 of the tubes 32 and 34 may be considered as connected in parallel. The circuit as it virtually exists for low frequency components is illustrated in FIG. 2.

One phase of the low frequency signal at the lead 80 is applied in parallel to the grids 38 and 90 of the tubes 32 and 34. For this purpose the lead 46 is joined to the lead 80 at a junction 92. The signal feeds through the resistors 33 and 40 to the respective grids 88 and 90. The judicious separate connection of the cathodes to ground independent of junction 42 ensures that this low frequency signal is not diverted from the grids. Also, it will be noted that the condenser 48 which grounds the medium and high frequency signals from junction 92 appears as an open circuit to the low frequency signals. A resistor 94 connected between the junction 92 and the ground connection 96 operates as a grid resistor for the tubes 32 and 34.

Similarly for the other phase, the lead 81 connects to the tubes 33 and 35 in parallel, reference characters for parts being numbered consecutively to those to which they correspond.

A low frequency transformer T3 has a primary winding 9? connected across the plates of the respective pushpull branches. For this purpose one terminal of the primary winding 99 connects to the center tap the transformer primary winding 82 as indicated by the lead lliyt). The opposite end of the transformer primary winding 99 connects by the aid of a lead 162 to a center tap of the transformer primary winding 83. The transformer secondary 106 drives a suitable low frequency speaker system.

The excitation for tubes 32, 34, 33 and 35 is provided via a center tap 103 in the transformer primary winding 99, as indicated by the legend Condensers 104 and provide midband shunts across the low frequency transformer T3 so that only low frequency energy drives the low frequency speaker system.

By the aid of the system described stereophonic signals above the low frequency levels, and significant for stereophonic effect, are delivered to appropriate reproducing systems. A single low frequency channel is provided with minimum circuitry. Expensive, high power rating filters for crossover networks are eliminated, and output transformers economically designed for specific frequency bands are provided.

The inventor claims:

1. In a stereophonic amplifier: a pair of power amplifiers for companion stereo channels and operable in the mid frequency range of the audio frequency spectrum; a pair of companion stereophonic input means for the power amplifiers respectively and through which information in the low frequency and mid frequency ranges is transmitted, and which are respectively connected to the power amplifiers; means deriving from the input means, and at points in advance of the power amplifiers, a composite signal corresponding to the sum of the low frequency components of said sources; and low output impedance coupling means interposed between said deriving means and the respective input means for maintaining isolation of the mid frequency signals in the respective channels.

2. In a stereophonic amplifier: a pair of power amplifiers for companion stereo channels and operable in the mid frequency range of the audio frequency spectrum; a pair of networks each providing a mid frequency range output and a low frequency range output; a pair of input means for conducting signals containing information in the low frequency and mid frequency range; means having low impedance relative to the networks for coupling the signal conducting means to the networks; means connecting the mid frequency range outputs of the networks respectively to the power amplifiers; and unitary means for amplifying the combined low frequency outputs of said networks.

3. The combination as set forth in claim 2 in which said low impedance means comprise a pair of cathode follower stages for the respective networks.

4. In a stereophonic amplifier: a pair of power amplifier stages designed for mid band application; each stage having a pair of electronic devices provided each with a cathode, an anode and a control electrode, said stage also having a mid frequency range transformer provided with a primary winding connected to the anodes of the devices to form a push-pull arrangement; means providing companion first and second mid range frequency stereo inputs; means providing a third combined low frequency input; means applying the first and second stereo input means to the respective amplifier stages for push-pull amplification; a low frequency power output transformer having a primary winding connected at one end to the transformer primary winding of one of said amplifier stages and connected at the other end to the transformer primary Winding of the other of said amplifier stages; means deriving a signal of one phase from said third input means and applying the same simultaneously and equally to the control electrodes of the devices of one of the amplifier stages; and means deriving a signal of the opposite phase from said third input means and applying the same simultaneously and equally to the control electrodes of the devices of the other of the amplifier stages whereby the two stages form respective pus'hapull branches for low frequency power amplification.

5. The combination as set forth in claim 4, together with a pair of input resistors for each stage, connected together at a junction and at places remote from said junction to the control electrodes of the corresponding stage; and means discriminatory to low frequency signals connecting said junction to the cathodes of the devices of said stages; the respective phases of the low frequency signal being applied to the input resistor junctions of the respective stages.

6. The combination as set forth in claim 4, in which said low frequency power output transformer primary is connected between mid-points of the primary windings of said amplifier stages, together with direct current mutation means for said devices, and connected thereto via a mid point of the primary Winding of said low frequency transformer.

7. The combination as set forth in claim 6, together with means for filtering mid frequency range signals from the primary winding of said low frequency transformers.

8. In a stereophonic amplifier: means providing a low frequency input; a first low frequency push-pull amplifier section; a second low frequency push-pull amplifier section; each push-pull section including a pair of electronic amplifying devices, a midrange operable power output means connected across said electronic devices, and providing a path between the devices that has a low impedance only as to low frequencies, and thereby connecting said electronic devices in parallel as to low frequencies but in push-pull as to midrange frequencies; means providing a pair of midrange frequency stereo inputs, one for the first push-pull section, and the other for the see ond push-pull section; means deriving one phase from the low frequency input means and applying the same substantially equally and simultaneously to the pair of electronic devices of said first low frequency push-pull amplifier section; means deriving another phase from the low frequency input means and applying the same substmtially equally and simultaneously to the pair of electronic devices of said second low frequency push-pull amplifier section; low frequency power output means connected at one end to the midrange operable power output means of said first push-pull amplifier section and at the other end to the midrange operable power output means of said second push-pull amplifier section; circuit means for deriving opposite phases from one midrange frequency stereo input means and applying the same respectively to the pair of devices of said first push-pull section; and circuit means for deriving opposite phases from the other midrange frequency stereo input means and applying the same respectively to the pair of devices of said second push-pull section.

References Cited in the file of this patent UNITED STATES PATENTS 2,273,866 Holst et a1. Feb. 24, 1942 2,520,798 Boer Aug. 29, 1950 2,714,633 Fine 1. Aug. 2, 1955 2,727,141 Cheek Dec. 13, 1955 2,757,571 Lo ughren Aug. 7, 1956 2,886,659 Schroeder May 12, 1959 OTHER REFERENCES Electronics World, June 1959, by Burstein, Phantom Channel for Stereo, pages 46, 47 and 80. 

1. IN A STEREOPHONIC AMPLIFIER: A PAIR OF POWER AMPLIFIERS FOR COMPANION STEREO CHANNELS AND OPERABLE IN THE MID FREQUENCY RANGE OF THE AUDIO FREQUENCY SPECTRUM; A PAIR OF COMPANION STEREOPHONIC INPUT MEANS FOR THE POWER AMPLIFIERS RESPECTIVELY AND THROUGH WHICH INFORMATION IN THE LOW FREQUENCY AND MID FREQUENCY RANGES IS TRANSMITTED, AND WHICH ARE RESPECTIVELY CONNECTED TO THE POWER AMPLIFIERS; MEANS DERIVING FROM THE INPUT MEANS, AND AT POINTS IN ADVANCE OF THE POWER AMPLIFIERS, A COMPOSITE SIGNAL CORRESPONDING TO THE SUM OF THE LOW FREQUENCY COMPONENTS OF SAID SOURCES; AND LOW OUTPUT IMPEDANCE COUPLING MEANS INTERPOSED BETWEEN SAID DERIVING MEANS AND THE RESPECTIVE INPUT MEANS FOR MAINTAINING ISOLATION OF THE MID FREQUENCY SIGNALS IN THE RESPECTIVE CHANNELS. 